To date, many underbody members, body structure members and the like of automobiles have been manufactured by performing press working on steel sheets having a specified strength. Nowadays, since there is a strong requirement to reduce the weight of an automobile body from the viewpoint of conservation of the global environment, efforts are being made to reduce the thickness of steel sheets used for automobile bodies by increasing the strength of the steel sheets. However, since an increase in the strength of steel sheets is accompanied by a decrease in press workability, there is an increase in the number of instances where it is difficult to form steel sheets into desired shapes for the members.
Therefore, UK Patent Publication No. GB 1490535 proposes a working technique called hot pressing which makes it possible to realize an increase in workability and an increase in strength at the same time by performing working and rapid cooling at the same time on a heated steel sheet using a mold composed of a die and a punch. However, in that hot pressing, since a steel sheet is heated at a high temperature of about 950° C. before hot pressing is performed, scale (iron oxide) is generated on the surface of the steel sheet, and flaking of the scale occurs when hot pressing is performed, which results in a problem in that a mold is damaged or in that the surface of a member is damaged after hot pressing has been performed.
Also, scale which is retained on the surface of a member results in a poor surface aesthetic appearance and causes a decrease in paint adhesiveness. Therefore, scale present on the surface of a member is usually removed by performing processing such as pickling and shot blasting. However, since such processing makes the manufacturing process complex, there is a decrease in productivity.
Moreover, the underbody members, body structure members and the like of automobiles are also required to have good corrosion resistance. However, since a hot-pressed member manufactured using the process described above is not provided with an anti-corrosion film such as a coating layer, the member is very poor in terms of corrosion resistance.
Therefore, since a hot pressing technique is required with which formation of scale can be suppressed when heating is performed before hot pressing is performed and with which the corrosion resistance of a hot-pressed member after hot pressing has been performed can be increased, a steel sheet to be hot-pressed whose surface is coated with a film such as a coating layer and a method of hot pressing which uses the steel sheet have been proposed. For example, Japanese Patent No. 3663145 discloses a method of manufacturing a hot-pressed member excellent in terms of corrosion resistance whose surface is coated with a Zn—Fe-based compound or a Zn—Fe—Al-based compound by performing hot pressing on a steel sheet which is coated with Zn or a Zn-based alloy.
In addition, in particular, to increase the paint adhesiveness of a galvanized steel sheet to be hot-pressed, Japanese Unexamined Patent Application Publication No. 2007-63578 discloses a galvanized steel sheet to be hot-pressed which is coated with a silicone resin film having a silanol group, and it is also said that the galvanized steel sheet is excellent in terms of phosphatability, after-painting corrosion resistance, and zinc volatility resistance.
However, in a hot-pressed member manufactured using the method according to Japanese Patent No. 3663145, a galvanized steel sheet or a zinc-aluminum-coated steel sheet having a low melting point is used. Therefore, since zinc undergoes an intense oxidation reaction on the surface of the coating layer in heating processing before hot pressing, a hot-pressed member obtained as a final product has insufficient paint adhesiveness. In addition, when the steel sheet to be hot-pressed described in Japanese Unexamined Patent Application Publication No. 2007-63578 is used, although there is an increase in the adhesiveness between a resin film, with which the surface of a coating layer is covered, and paint, since the galvanizing layer undergoes an intense oxidation under some heating treatments before hot pressing is performed, it is difficult to reliably achieve satisfactory paint adhesiveness.
It could therefore be helpful to provide a hot-pressed member excellent in terms of paint adhesiveness and a method of manufacturing the hot-pressed member.